PDE4A and psoriatic arthritis: Since the marketing of Roflumilast, three additional compounds have been marketed as PDE4-inhibitor drugs (Figure 4) i.e., Crisaborole (by Pfizer) for atopic dermatitis [71], Apremilast (by Celgene) for psoriatic arthritis [72] and Ibudilast (by MediciNova) for Krabbe diseases (also known as globoid cell leukodystrophy) [73].